Assembly of cytoplasmic microtubules from brain is being investigated by small angle light scattering and by ultracentrifugation. In addition, the newly discovered physical and enzymic interactions between microtubules and neurofilaments are being studied. The two formed organelles associate in solution to form a complex of high viscosity. Neurofilament-associated protein kinase is stimulated when the microtubule-associated protein MAP-2 is present in solution. An investigation of the mechanism of this stimulation is being undertaken, in concert with an investigation of the consequences of phosphorylation of microtubule proteins by the neurofilament-associated protein kinase.